This invention relates to a system and apparatus for generating an optical noise in a predetermined bandwidth. This optical noise can be used in many applications, such as photodetector calibration and white light spectroscopy. In photodetector calibration the optical noise output, which is relatively flat over a certain bandwidth, is sent to a photodetector. The photodetector's electrical response is then examined on a spectrum analyzer to find distortions which may be caused by the frequency response of the photodetector. In white light spectroscopy, the optical noise output is sent to the material being tested and the absorption spectrum is analyzed.
Prior art optical noise generators include two-pass noise generators which use amplified spontaneous emission (ASE). "High Power Compact 1.48 .mu.m Diode Pumped Broadband Superfluorescent Fibre Source at 1.58 .mu.m"; H. Fevrier, et al.; Electronic Letters, Vol. 27, No. 3; Jan. 31, 1991; gives an example of such a noise generator. This article discloses the use of the optical amplifier in a two-pass noise generator as shown in FIG. 2. The optical amplifier 10 may consist of a doped amplifying fiber 16 used as the gain medium and a pumping laser 12 which sends optical energy to the doped amplifying fiber via a wavelength division multiplexer (WDM) 14.
Optical noise is spontaneously emitted in the doped amplifying fiber 16 powered by the pumping laser 12. In the system disclosed by Fevrier, optical noise created by spontaneous emission travels through the doped amplifying fiber and is amplified. The amplified optical noise components then go to a mirror 18 which reflects the amplified optical noise components back to the optical amplifier. The optical amplifier amplifies the optical noise components a second time, and then the twice-amplified optical noise components travel to the output.
The pumping frequency of the pumping laser 12 is chosen so that the frequency is absorbed by the doped amplifying fiber 16. The energy from the pumping laser 12 goes through the wavelength division multiplexer 14 to pump the doped amplifying fiber 16 to a higher energy state, so that the doped amplifying fiber 16 will amplify optical signals such as optical noise components coming in through the optical path, and so that the doped amplifying fiber 16 will spontaneously emit light energy.
Looking at FIG. 2, the wavelength division multiplexer 14 works by multiplexing the pumping frequency on line B onto line C, so that the doped amplifying fiber 16 can absorb the pumping frequency and amplify the optical signal on the path. Signals going into the wavelength division multiplexer (WDM) 14 from line C will be de-multiplexed into two signals: on line A, the signal which contains the optical noise components not within the pumping frequency; and on line B, the optical signals of the pumping frequency are sent back to the pumping laser.
Other similar prior art systems use a filter at the output of the noise generator so that the optical noise components will be within a desired predetermined bandwidth. Because the filter is placed at the output of the optical path, the optical amplifier amplifies optical noise components that are not within the predetermined bandwidth, during the second amplification of the optical noise components. This unnecessary amplification of optical noise components outside the predetermined bandwidth may cause the optical amplifier to saturate. If the amplifier saturates, the optical noise components within the predetermined bandwidth are not amplified as much as the components would be amplified if the optical amplifier were unsaturated. Additionally, amplifying the optical noise components outside the predetermined bandwidth expends pump power from the pumping laser 12.
It is therefore an object of the present invention to provide a noise source that efficiently uses pump power.
A further object of the invention is to have a noise source that concentrates the available noise power in a narrow optical bandwidth.